A structural study of the (Na1-xKx)0.5Bi0.5TiO3 perovskite series as a function of substitution (x) and temperature

Rietveld neutron powder profile analysis of the (Na1-xKx)(0.5)Bi0.5TiO3 (NKBT) series (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1.0) is reported over the temperature range 293-993 K. A detailed characterization of the structures and phase transitions occurring across this series as a function of temperature has been made. Room-temperature refinements have revealed a rhombohedral phase, space group R3c for x = 0, 0.2, and 0.4, which exhibits an antiphase, a (-) a (-) a (-) oxygen tilt system with parallel cation displacements along [111](p). An intermediate zero-tilt rhombohedral phase, space group R3m possessing cation displacements along [111]p, has been established for x = 0.5 and 0.6. At the potassium-rich end of the series at x = 0.8 and 1.0, a tetragonal phase, space group P4mm is observed possessing cation displacements along [001]. At the sodium-rich end of the series for x = 0.2, the unusual tetragonal structure with space group P4bm is seen for Na0.5Bi0.5TiO3 which possesses a combination of in-phase a(0)a(0)c(+) tilts and antiparallel cation displacements along the polar axis. Temperature-induced phase transitions are reported and structural modifications are discussed. (C) 2002 International Centre for Diffraction Data.